1. Field of the Invention
The present invention relates to optical pickup apparatuses which read/write information signals from/to optical recording media, such as optical disks, magnetic optical disks, optical cards, etc., and to read/write apparatuses which contain the optical pickup apparatuses and which read/write information signals from/to optical recording media.
2. Description of the Related Art
Optical pickup apparatuses which read/write information signals from/to optical recording media, such as optical disks, etc., have been suggested. In such optical pickup apparatuses, in order to increase the storage capacities of the optical recording media, a laser diode (LD) having a short emission wavelength is often used as a light source.
In addition, in the optical pickup apparatuses, a lens having a high numerical aperture (NA) is used as an objective lens.
For example, compared with optical pickup apparatuses for xe2x80x9cCompact Discs (CDs)xe2x80x9d (trademark), in which the emission wavelength of the light source is 780 nm, in optical pickup apparatuses for xe2x80x9cDigital Versatile Discs (DVDs)xe2x80x9d (trademark), the emission wavelength of the light source is 650 nm. Thus, the emission wavelength of the light source is reduced as the recording density is increased. In addition, the numerical aperture (NA) of the objective lens used in the optical pickup apparatuses for Compact Discs (CDs) (trademark) is 0.45, and the numerical aperture (NA) of the objective lens used in the optical pickup apparatuses for Digital Versatile Discs (DVDs) is 0.60. Thus, the numerical aperture (NA) of the objective lens is increased as the recording density is increased.
Recently, an optical pickup apparatus using a blue-violet laser diode, of which the emission wavelength is 405 nm, as the light source and an objective lens having a high numerical aperture (NA) of 0.85 has been suggested.
However, in the case in which the emission wavelength of the light source is reduced and the numerical aperture (NA) of the objective lens is increased, there is a problem in that a wavefront aberration is easily increased due to various manufacturing errors, and the optical performance is degraded.
Accordingly, in order to correct a spherical aberration which occurs due to a thickness error of an optical recording medium and manufacturing errors in the lens, an optical pickup apparatus in which an expander lens unit including two lenses is inserted between the light source and the objective lens has been suggested. In this optical pickup apparatus, the distance between the two lenses included in the expander lens unit can be adjusted.
In addition, the spherical aberration can also be corrected by moving a collimator lens unit, which is used for making a light beam emitted from the light source parallel, along the optical axis.
However, the above-described construction in which the expander lens unit is used for correcting the spherical aberration has the following problems.
In the above-described expander lens unit, when the distance between the two lenses is changed from a designed value, that is, a value at which the light beam emitted from the expander lens unit is made parallel, the light beam emitted from the expander lens unit is either diffused or converged. The total quantity of light incident to the objective lens is limited by an aperture diaphragm disposed near the objective lens. Accordingly, compared with the case in which the light beam is parallel, the total quantity of light incident to the objective lens is increased when the light beam is converged and is reduced when the light beam is diffused.
When the total quantity of light incident to the objective lens changes, the quantity of light radiated on an optical recording medium in the process of reading/writing information signals from/to the optical recording medium also changes. In addition, the quantity of light that is reflected from the optical recording medium and detected by a detector also changes. Accordingly, there is a risk that error signals for implementing various servo operations cannot be adequately detected. In addition, even if the servo operations are stable, in the process of reproducing so-called xe2x80x9cRF signalsxe2x80x9d, that is, information signals read out from the optical recording medium, the xe2x80x9cRF amplitudexe2x80x9d also changes so that data may not be read accurately.
Furthermore, in the case in which the optical recording medium is a so-called xe2x80x9crecordable discxe2x80x9d, when the quantity of light radiated on the optical recording medium is too high, there is a risk that the information signals recorded on the optical recording apparatus will be destroyed in the reproducing process. On the contrary, when the quantity of light radiated on the optical recording medium is too low, there is a risk that the information signals cannot be recorded on the optical recording medium in the recording process.
The quantity of light is not the only problem. Generally, the light beam incident to the aperture diaphragm of the objective lens is a so-called Gaussian beam, in which the cross-sectional distribution of light intensity has the form of a Gaussian distribution. However, when the light beam incident to the aperture diaphragm is converged or diffused, the used area of the light beam changes. Thus, the so-called xe2x80x9crim intensity (RI)xe2x80x9d, that is, the light intensity of the peripheral region of the light beam relative to that of the central region thereof, also changes, so that the numerical aperture (NA) of the optical system changes substantially. In such a case, there is a risk that it will be difficult to read/write the information signals from/to the optical recording medium.
The above-described problem also occurs in the case in which the spherical aberration is corrected by moving the collimator lens unit along the optical axis.
In view of the above-described situation, an object of the present invention is to provide an optical pickup apparatus in which the quantity of light incident to the aperture diaphragm of the objective lens and the xe2x80x9crim intensity (RI)xe2x80x9d is maintained constant, even when the spherical aberration is corrected by passing the light beam through an expander lens unit before it reaches the objective lens, and which is able to adequately read/write information signals from/to optical recording media and to provide a read/write apparatus which includes the optical pickup apparatus.
In order to attain the above-described object, according to one aspect of the present invention, an optical pickup apparatus includes a light source, an objective lens which condenses a light beam emitted from the light source on an optical recording medium, and an expander lens unit which includes a first lens group and a second lens group, which is disposed on the optical axis between the objective lens and the light source, and in which the distance between the first and second lens groups is changed so as to correct a spherical aberration which occurs on the optical recording medium. The expander lens unit is disposed such that the focal position of the second lens group, which is closer to the objective lens than the first lens group, at the side closer to the objective lens is on the aperture diaphragm of the objective lens.
Thus, in this optical pickup apparatus, the distance between the expander lens unit and the aperture diaphragm of the objective lens is determined such that the focal position of the second lens group, which is closer to the objective lens than the first lens group, is on the aperture diaphragm of the objective lens. Even when the distance between the first and second lens groups is changed and the light beam emitted from the expander lens unit is thereby diffused or converged, the diameter of the light beam is always the same at the focal position of the second lens group.
At positions away from the focal position of the second lens group, the diameter of the light beam changes as the distance between the first and second lens groups is changed. Accordingly, when the aperture diaphragm of the objective lens is disposed at a position away from the focal position of the second lens group, the quantity of light incident to the aperture diaphragm changes when the spherical aberration is corrected by the expander lens unit.
The distance between the objective lens and the second lens group varies due to a focus servo operation in which the objective lens is moved. However, since a stroke of a focus actuator is normally approximately 2 mm (xe2x96xa1}1 mm), when the focal distance of the second lens group is sufficiently large, the change in quantity of light that occurs due to the movement of the objective lens can be ignored.
When the distance between the first and second lens groups of the expander lens unit is adjusted to correct the spherical aberration, it is better to move the first lens group, which is farther from the objective lens, than to move the second lens group. This is because the distance between the aperture diaphragm and the second lens group is preferably maintained as constant as possible.
Each of the first and second lens groups of the expander lens unit may be constructed of a single lens or a plurality of lenses. In the case in which the second lens group is constructed of a plurality of lenses, it should be disposed such that the focal position of the combination of the lenses is on the aperture diaphragm of the objective lens.
In addition, according to another aspect of the present invention, an optical pickup apparatus includes a light source, an objective lens which condenses a light beam emitted from the light source on an optical recording medium, and a collimator lens unit which is disposed on the optical axis between the objective lens and the light source. The collimator lens unit is disposed such that the focal position at the side closer to the objective lens is on the aperture diaphragm of the objective lens.
The above-described descriptions regarding the expander lens unit also apply to the case in which the collimator lens unit is moved along the optical axis so as to correct the spherical aberration. In this case, the aperture diaphragm of the objective lens is disposed on the focal position of the collimator lens unit.
Focal distances of the collimator lens units are often limited by the characteristics of a laser diode (LD) that serves as a light source and by the size of an optical system. In such a case, in the above-described optical pickup apparatus, the position for disposing the aperture diaphragm of the objective lens is also limited. In order to eliminate such a limit, the collimator lens unit may be constructed of two or more lens groups in which the distance between one of the lenses which is closest to the objective lens and the rest of the lenses can be changed, and the spherical aberration may be corrected by adjusting this distance. In this case, the focal distance of the lens which is closest to the objective lens can be freely determined, so that the position to dispose the aperture diaphragm of the objective lens can also be freely determined.
Accordingly, the quantity of light incident to the objective lens can be made constant irrespective of the state of the expander lens unit or the collimator lens unit, and the quantity of light detected by a detector can be stabilized. Accordingly, servo-error signals can be adequately detected. In addition, in the process of reproducing xe2x80x9cRF signalsxe2x80x9d, the xe2x80x9cRF amplitudexe2x80x9d can also be stabilized so that data can be accurately reproduced. Furthermore, since the quantity of light radiated on the optical recording medium is stabilized, information signals can be adequately read/written from/to the optical recording medium.
Furthermore, since the diameter of the light beam, which is increased by the above-described expander lens unit, is constant at the focal position of the second lens group irrespective of the state of the expander lens unit, the xe2x80x9crim intensity (RI)xe2x80x9d and the numerical aperture (NA) do not change.
Thus, according to the optical pickup apparatus of the present invention, even when the spherical aberration is corrected using the expander lens unit or the collimator lens unit, the change in quantity of light and xe2x80x9crim intensity (R)xe2x80x9d can be prevented.
In addition, according to another aspect of the present invention, a read/write apparatus comprises an optical pickup apparatus, and an optical recording medium supporting unit which supports an optical recording medium in such a manner that the optical pickup apparatus is able to read/write information signals from/to the optical recording medium. The optical pickup apparatus includes a light source, an objective lens which condenses a light beam emitted from the light source on the optical recording medium, and an expander lens unit which includes a first lens group and a second lens group, which is disposed on the optical axis between the objective lens and the light source, and in which the distance between the first and second lens groups is changed so as to correct a spherical aberration which occurs on the optical recording medium, and the expander lens unit is disposed such that the focal position of the second lens group, which is closer to the objective lens than the first lens group, at the side closer to the objective lens is on the aperture diaphragm of the objective lens.
In addition, according to another aspect of the present invention, a read/write apparatus comprises an optical pickup apparatus, and an optical recording medium supporting unit which supports an optical recording medium in such a manner that the optical pickup apparatus is able to read/write information signals from/to the optical recording medium. The optical pickup apparatus includes a light source, an objective lens which condenses a light beam emitted from the light source on the optical recording medium, and a collimator lens unit which is disposed on the optical axis between the objective lens and the light source, and the collimator lens unit is disposed such that the focal position at the side closer to the objective lens is on the aperture diaphragm of the objective lens.
Thus, according to the present invention, when the spherical aberration is corrected by moving one of the lenses in the expander lens unit or the collimator lens unit, the quantity of light incident to the objective lens can be maintained constant irrespective of the operation for correcting the spherical aberration. In addition, since a large aberration can also be corrected, the tolerance for the thickness error of the recording media and manufacturing errors in the objective lens can be increased.
In addition, information signals can be adequately and reliably read/written from/to the optical recording medium without adjusting the quantity of light emitted from the light source.
More specifically, according to the present invention, an optical pickup apparatus, in which the quantity of light incident to the aperture diaphragm of the objective lens and the xe2x80x9crim intensity (RI)xe2x80x9d are maintained constant even when the spherical aberration is corrected by passing the light beam through an expander lens unit before it reaches the objective lens, and which is able to adequately read/write information signals from/to optical recording media, and a read/write apparatus which includes the optical pickup apparatus can be provided.